1. Field of the Invention
The present invention relates to a method of producing a hot-stamped article formed by hot stamping on which a coating treatment can be suitably performed.
2. Description of Related Art
Recently, in the automobile industry, a high tensile strength steel sheet has been commonly used for improving collision safety and reducing the weight of a vehicle to improve fuel efficiency. However, when the strength of steel to be used increases, galling or rupture of a steel sheet may occur during press-forming, or the shape of a formed article may be unstable due to the spring-back phenomenon.
As a technique of producing a high-strength article, hot press-forming (hot stamping) in which a high strength steel sheet is not simply pressed (cold pressing) but is press-formed in a low-strength state such as a heating state is adopted.
In hot stamping, a steel sheet is pressed in a state of being heated to an austenite transformation temperature range of 800° C. or higher to be softened. At the same time, the quenching of the steel sheet is strengthened with a cooling effect (contact cooling) obtained by contact with dies. As a result, since the strength of a pressed article is improved and the residual stress thereof decreases, the sensibility of season cracking, delayed fracture, and the like which are problems of a high tensile strength steel sheet also decreases.
However, a galvannealed steel sheet is superior in strength, weldability, coating properties, and the like and thus is used as a steel sheet for a vehicle body. When such a galvannealed steel sheet is used, a coating treatment is performed on the surface thereof.
In this coating treatment, for example, phosphating is performed as a surface treatment, and a coating film is formed on a phosphate crystal film. In order to perform phosphating, it is important to form a superior phosphate crystal film from the viewpoints of securing superior coating performance such as adhesion and corrosion resistance of a coating film. In particular, a galvannealed steel sheet is formed of a Zn—Fe alloy, which is highly reactive with a phosphating solution, and contains substantially no impurities and the like. Thus, a superior phosphate crystal film can be formed.
However, even in this case, an oxide is formed on the surface of the steel sheet because hot stamping is typically performed in the atmosphere. This oxide is formed as a zinc-based oxide film. When this oxide film is formed, the adhesion of a coating film (in particular, the adhesion of a phosphate crystal film) may decrease.
From this point of view, for example, a method of producing a hot-stamped article is disclosed, the method including: a hot press-forming process of performing hot press-forming (hot stamping) on a galvanized steel sheet on which a galvanized layer is formed; and a zinc oxide layer removal process of removing, by shot blasting, a zinc oxide layer of the galvanized layer which is formed on the surface of the galvanized steel sheet through a heating process (for example, refer to Japanese Patent Application Publication No. 2012-25977 (JP 2012-25977 A).
In this way, the adhesion of a coating film can be improved by removing a zinc oxide layer (oxide film). However, according to an experiment described below by the present inventors, there may be a case where, even if the above-described shot-blasting is performed to remove an oxide film, a part of the oxide film is crushed and remains in a steel sheet. As a result, the adhesion of a coating film may be insufficient. In addition, a part of a plating film may be peeled off and corrosion protection performance of the plating film may decrease. In addition, compressive residual stress is applied to a product by collision between shot particles, which may deform the product.